1. Field of the Invention
The present invention relates to a focus detector for use in optical instruments such as cameras. More particularly, the present invention relates to such type of focus detector in which a pair of refocusing (image-reforming) optical systems and a pair of photo receptors are provided to detect the focus of a main image-forming optical system. An object image formed by the main image-forming optical system is refocused, namely image-reformed on the pair of photo receptors through the pair of refocusing systems respectively so that the focus of the main image-forming optical system can be detected by detecting the existing relative positional relationship between the refocused images on the pair of photo receptors.
2. Description of the Prior Art
A known optical arrangement of the above-mentioned type of focus detector for cameras is shown in FIG. 1 in which FIG. 1A is a front view of the optical system of the prior art focus detector for cameras and FIG. 1B is a plan view thereof.
In FIG. 1, 1 is a photographic lens and 2 is a predetermined focal plane of the lens 1. A field lens 3 is disposed in or near the predetermined focal plane 2. The predetermined focal plane 2 is located at or near a position conjugated with a film. 4A and 4B are a pair of refocusing lenses. 5 denotes a plane conjugated with the predetermined focal plane 2 in regard to the pair of refocusing lenses 4A and 4B. On the conjugate plane 5 there are image position detecting photo-electric sensors 6A and 6B. The photographic lens 1 forms an image of an object on the predetermined focal plane 2. Each of the refocusing lenses 4A and 4B forms a secondary image of the object image on the conjugate plane 5. For the purpose of this specification, the predetermined focal plane 2 is referred to as the primary image plane and the conjugate plane 5 as the secondary image plane.
2A denotes a rectangular area in the primary image plane 2. The center of the rectangular area 2A lies on the optical axis 0 of the photographic lens. This area 2A is an area used for the detection of focus. Therefore, this area 2A is referred to as the primary image plane detection area. Areas 5A and 5B on the secondary image plane 5 are conjugate with the primary image plane detection area 2A. The conjugate area 5A or 5B is referred to as the secondary image plane detection area. The secondary image plane detection area 5A is coincident with the light reception surface of the photo-electric sensor 6 and the secondary image plane detection area 5B is coincident with the light reception surface of the photo-electric sensor 6B. When the photographic lens 1 is moved along the optical axis 0, the object image formed by the lens 1 moves also along the optical axis 0. Consequently, the secondary images formed by the refocusing lenses 4A and 4B move on the secondary image plane at the same time. The state of the focus adjustment of the photographic lens 1 can be detected by detecting the relative positions of the secondary images on the photo-electric sensors 6A and 6B.
However, this type of prior art focus detector has an important disadvantage in that it needs a very voluminous optical system 7 as indicated by the broken line in FIG. 1A. The volume of the focus detection optical system 7 is too large to as received in the camera body.
To reduce the size of the focus detector it has already been proposed to replace the refocusing lenses by concave mirrors. A typical example of such a reflection type focus detection optical system is disclosed, for example, in the specification of U.S. Pat. No. 4,384,770. The basic arrangement of this reflection type focus detection optical system will hereinafter be described with reference to FIG. 2.
In FIG. 2, the concave mirrors 8A and 8B are behind the above-mentioned rectangular primary image plane detection area 2A and are disposed approximately symmetrically relative to the optical axis of the photographic lens 1. 9A and 9B denote the secondary image plane detection areas in which secondary images are formed by the concave mirrors 8A and 8B respectively. In this case, it is required to avoid the overlap of the secondary image plane detection areas 9A, 9B and the primary image plane detection area 2A. To this end, the optical axis of each concave mirror is inclined relative to the plane defined by the center of the primary image plane detection area and the centers of areas of the concave mirrors 8A and 8B. Herein the optical axis of the concave mirror is defined as the normal line extending from the center area of the mirror. More specifically, the optical axis of the concave mirror 8A is inclined down by an angle .phi. relative to the plane defined by the three centers whereas the optical axis of the concave mirror 8B is inclined up by .phi. relative to the plane as seen in FIGS. 2B and C. By this inclination, the secondary image plane detection area 9A relating to the concave mirror 8A is formed in an area downwardly spaced from the primary image plane detection area 2A and the secondary image plane detection area 9B relating to the concave mirror 8B is formed in an area upwardly spaced from the primary image plane detection area 2A. To detect the displacement of the secondary image, a photo-electric sensor device is provided in each the secondary image plane detection areas 9A and 9B.
With the above arrangement, the reflection type focus detector has a size considerably smaller than the first mentioned prior art focus detector having a pair of refocusing lenses. However, this reflection type focus detector has a problem in that the quality of the secondary image formed therein is degraded due to the inclined arrangement of the concave mirrors. There is the possibility that the identity of the secondary image with the primary one may be impaired to a great extent. Hereinafter a further detailed explanation of the problem will be made with reference to FIG. 2.
Regarding the rays of light coming from the center of the primary image plane detection area 2A to impinge upon the centers of the concave mirrors 8A and 8B and then to reach the centers of the secondary image plane detection areas 9A and 9B, the sum of the incident angle to the concave mirror and the reflection angle by the mirror is referred to as the deflection angle. As will be understood from FIGS. 2B and C, the deflection angle is 2.phi., that is, twice the angle of the above-described inclination .phi.. This deflection angle has a large adverse effect on the image-forming performance of the concave mirror. The aberration of the concave mirror varies in magnitude with the deflection angle. More concretely, coma increases in proportion to the deflection angle .phi.. Astigmatism increases in proportional to .phi..sup.2. As will be described in detail later, the deflection angle of beams coming from points other than the center of the primary image plane detection area 2A becomes larger with the distance from the center. Therefore, the degradation of the secondary image caused by aberration becomes most severe at the edge portion of the area.
In this manner, in the case of the prior art reflection type focus detection optical system, the astigmatism of the concave mirror is greatly increased with increasing inclination of the mirror, thereby degrading the secondary image to a great extent. The accuracy of detection of the relative position of the secondary images by the pair of photo-electric sensors is greatly reduced thereby. With this type of prior art focus detector, therefore, it is impossible to attain the focus detection with desired high accuracy.
For photo-electrical detection bright secondary images which have high illuminance are required. Generally, a bright secondary image can be obtained by use of a concave mirror having a larger diameter. However, the use of such a concave mirror having a larger diameter exacerbates the problem of the degradation of the secondary image as described above.